1. Field of the Invention
The present invention relates to an ion implantation method and more specifically to a method for manufacturing a semiconductor device using the ion implantation method.
2. Description of the Related Art
An ion implantation method is one of doping methods of an impurity. Compared to other doping methods such as a doping method during film formation and a thermal diffusion method, the ion implantation method has an extremely large ion implantation amount and extremely high control accuracy of an implantation depth, and has a merit capable of low concentration control. Dopant used for a semiconductor device can be roughly classified into n-type dopant and p-type dopant. A Group 5 element such as phosphorus (P), arsenic (As), or antimony (Sb), a Group 6 element such as sulfur (S), tellurium (Te), or selenium (Se), or the like that functions as a donor is known as an impurity element imparting n-type conductivity, and P is mainly used in an industry. In addition, a Group 3 element such as boron (B), aluminum (Al), gallium (Ga), or indium (In), a Group 2 element such as zinc (Zn), or the like that functions as an acceptor is known as an impurity element imparting p-type conductivity, and B is mainly used in an industry.
Ion implantation is carried out in a chamber by generating an ion from an ion source, extracting an ion by applying an electric field, splitting an ion that is needed with an analyzing magnet, accelerating the ion in electrostatic up to an acceleration energy that is needed, and implanting the ion into a semiconductor film formed over a substrate.
Usually, loading and unloading of a substrate to a chamber in an ion implantation apparatus is carried out through a load lock chamber. Therefore, the apparatus can be operated without exposing the chamber to the atmosphere. However, it is necessary to expose the chamber to the atmosphere regularly for, for example, maintenance. In this case, the inside of the chamber is replaced by a doping gas after the chamber is exposed to an atmosphere; however, a constituent such as water adhered to the inside of the chamber by being exposed to the atmosphere is remained for a certain periods after operating the apparatus. Thus, treatment over a substrate has to be refrained until an ion species ratio is stabilized and there is a problem that a long down time ranging from 40 hours to 50 hours is needed for aging.
In addition, compared to P, B generally has a low ion species ratio at the time of ion implantation; therefore, B is inferior in respect of throughput. Although it is possible to raise the ion species ratio of B to some extent by increasing a flow or a concentration of a gas that serves as dopant, it is still inadequate to obtain throughput equivalent to P. Moreover, compared to P, B is subject to variation since the ion species ratio is low; therefore, there is a problem that a longer down time is needed to stabilize the ion species ratio after the chamber is exposed to the atmosphere.